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    <title>总结</title>
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    Scroll: 滚动家族
        Width/Height : 获取元素内容大小 width/height + padding  意外：内容超出，只读，且为整数
        Top/Left：获取元素被滚动条卷入的上左边距， 可读写的非整数值
        监听：elem.onscroll
    页面：
        获取卷入上左边距：html.scrollTop/Left || window.pageY/XOffset || body.scrollTop/Left  
        设置卷入：window.scroll(x,y)  window.scrollBy(x,y)
        监听滚动：window.onscroll = func
    顺滑滚动：
        1. html { scroll-behavior: smooth }
        2. window.scroll/By({ top:xx , left: xx, behavior: "smooth" })

    练习--滚动相关的场景
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